The present invention relates to a transmission interruption signal detection system of a facsimile system and more particularly to a transmission interruption signal detection system for detecting a transmission interruption signal at a transmission apparatus of facsimile system when the reception apparatus is hung up during the transmission of image information from the transmission apparatus to a reception apparatus of the facsimile system.
In the facsimile system when a reception apparatus is hung up for some reason and the reception of image information is discontinued or interrupted during the transmission of image information from the transmission apparatus to the reception apparatus, some measure has to be taken by detecting a signal indicating the interruption of the transmission.
An image information communication system capable of taking a measure against such interruption of communication by detecting a transmission interruption signal at a transmission apparatus of the image information communication system has been proposed in the U.S. Pat. No. 4,112,467 assigned to Ricoh Company Ltd.
In the above-mentioned invention, when the interruption signal is set at a predetermined frequency, there is a risk that the transmission apparatus detects, as the transmission interruption signals, image information components having the same frequency as that of the transmission interruption signal, which are produced from the transmission apparatus itself, so that malfunction of the transmission apparatus occurs.
According to a standard G 3 made by international Telegraph and Telephone Consultative Committee (CCITT), it is required that when reception apparatus of facsimile system is hung up, the reception apparatus generates a transmission interruption signal of 462 Hz for 3 seconds for indicating interruption of communication and send the signal to its counterpart transmission apparatus.
However, since the transmission interruption signal of 462 Hz in confirmity with the standard of CCITT is of a low frequency, the signal is attenuated considerably in the channel connecting the transmission apparatus to the reception apparatus due to the low frequency characteristic of telephone line. For example, when the transmission apparatus is transmitting image information at -7 dBm, the transmission apparatus has to detect a transmission interruption signal, attenuated to as low as -30 dBm, which is transmitted from the reception apparatus through the same channel.
Furthermore, since the transmission frequency of image information contains the same frequency component as that of the transmission interruption signal, the detection of the transmission interruption signal transmitted from the reception apparatus to the transmission apparatus becomes the more difficult.
For examole, the frequency band width of 4800 BPS Modem is approximately in the range from 600 Hz to 2700 Hz, and the frequency band width of 9600 BPS Modem is approximately in the range from 500 Hz to 3000 Hz. Both Modems generate the low frequency components of 462 Hz intermittently. Therefore, it is extremely difficult for the transmission apparatus to detect the transmission interruption signal of 462 Hz, which is sent from the reception apparatus to the transmission apparatus through the same channel during the transmission of the image information, by use of only a frequency detection circuit. In particular, since the lowermost frequency of 9600 BPS Modem is closer to 462 Hz than that of 4800 PBS, 9600 BPS Modem frequently generates more component signals having the same frequency as that of the transmission interruption signal. Therefore, there is a risk that the 462 Hz signal components of the image information produced from the transmission apparatus are detected erroneously as the transmission interruption signal, so that a malfunction occurs in the transmission apparatus.
Therefore, in the case of 4800 BPS Modem, even if a transmission interruption signal of -30 dBm sent from the reception apparatus is detected successfully by the transmission apparatus, in the case of 9600 BPS Modem, the sole detection of the transmission interruption signal becomes more difficult since the lowermost detectable limit of 9600 BPS Modem is, for example, -25 dBm.